Note: Cara Cara oranges are a type of navel orange with reddish flesh; if your local market doesn't have them, use regular oranges instead. Cotija cheese is also widely available, but if you cannot find it, substitute Feta or ricotta salata.

Preheat oven to 350°. Spread pepitas in a single layer on a baking sheet, and toast in oven until crisp, about 8 minutes. (Alternatively, toast them in a dry skillet over medium heat until crisp, 6 to 8 minutes, stirring occasionally to keep them from burning.) Let cool in pan.

Meanwhile, cut peel and pith from 2 oranges; slice thinly and set aside. Juice remaining 1/2 orange. Make a vinaigrette by whisking together orange juice, shallot, mustard, salt, grapeseed oil, and dill or chives (if using) in a medium bowl.

To assemble salad: Mix mâche and tatsoi; toss lightly with most of vinaigrette. Add oranges and avocado slices; sprinkle pepitas and cheese on top. Serve with remaining vinaigrette on the side.